Recent years have seen a rapid expansion in the performance of solid state lighting devices such as light emitting devices (LEDs); and with improved performance, there has been an attendant expansion in the variety of applications for such devices. For example, rapid improvements in semiconductors and related manufacturing technologies are driving a trend in the lighting industry toward the use of light emitting diodes (LEDs) or other solid state light sources to produce light for general lighting applications to meet the need for more efficient lighting technologies and to address ever increasing costs of energy along with concerns about global warming due to consumption of fossil fuels to generate energy. LED solutions also are more environmentally friendly than competing technologies, such as compact florescent lamps, for replacements for traditional incandescent lamps.
The actual solid state light sources, however, produce light of specific limited spectral characteristics. To obtain white light of a desired characteristic and/or other desirable light colors, one approach uses sources that produce light of two or more different colors or wavelengths and one or more optical processing elements to combine or mix the light of the various wavelengths to produce the desired characteristic in the output light. One technique involves mixing or combining individual light from LEDs of three or more different wavelengths (spectral colors such as “primary” colors), for example from Red (R), Green (G) and Blue (B) LEDs. With a LED-centric approach such as LED based RGB, the individual color amounts can be adjusted easily to a wide range of colors, including different color temperatures of white light, in the fixture output. There are applications where the ability to adjust or ‘tune’ the color of white light is desirable. However, with the approach using LEDs of three different monochromatic colors, the output spectrum tends to have a small number of narrow spikes, which produces a low color rendering index (CRI). An LED system can actually be designed to somewhat mimic a desired CRI rating, by careful selection of the LED colors to meet the CIE color test components, yet the LED light output may provide less than optimal illumination of some colors on objects or in areas illuminated by the LED lighting system. It is possible to improve the CRI by providing additional LEDs of different colors, but that approach increases complexity and overall system cost.
Another LED-centric approach to white lighting combines a white LED source, which tends to produce a cool bluish light, with one or more LEDs of specific wavelength(s), such as red and/or yellow, chosen to shift a combined light output to a more desirable color temperature. Adjustment of the LED outputs offers control of intensity as well as the overall color output, e.g. color and/or color temperature of white light. However, even this approach may have some narrow spiking in the emission spectrum, e.g. due to the red and/or yellow LED light used to correct the color temperature, and as a result, the color rendering may still be less than desirable.
In recent years, techniques have also been developed to shift or enhance the characteristics of light generated by solid state sources using phosphors, including for generating white light using LEDs. Phosphor based techniques for generating white light from LEDs, currently favored by LED manufacturers, include UV or Blue LED pumped phosphors. In addition to traditional phosphors, semiconductor nanophosphors have been used more recently. The phosphor materials may be provided as part of the LED package (on or in close proximity to the actual semiconductor chip), or the phosphor materials may be provided remotely (e.g. on or in association with a macro optical processing element such as a diffuser or reflector outside the LED package). The remote phosphor based solutions have advantages, for example, in that the color characteristics of the fixture output are more repeatable, whereas solutions using sets of different color LEDs and/or lighting fixtures with the phosphors inside the LED packages tend to vary somewhat in light output color from fixture to fixture, due to differences in the light output properties of different sets of LEDs (due to lax manufacturing tolerances of the LEDs).
However, where some control of color characteristic is provided, it is provided by additional dynamically controllable LEDs. The controlled LEDs used for tuning may be specific color LEDs or substantially white LEDs of one or more color temperatures selected to adjust the light color characteristic of light produced by pumping of the phosphor. Like the LED-centric tuning of the white LED with a specific color, however, LED centric tuning of the phosphor emissions may have some narrow spiking in the emission spectrum, and as a result, the color rendering may still be less than desirable.
Solid state lighting technologies have advanced considerably in recent years, and such advances have encompassed any number of actual LED based products, however there is still room for further improvement in the context of lighting products. For example, it is desirable to provide a light output spectrum that generally conforms to that of the lighting fixture or lamp the solid state lighting device may replace. As another example, it may be desirable for the solid state lighting device to provide a tunable color light output of color. It may also be useful for such a device to provide intensity and output distribution that meet or exceed expectations arising from the older replaced technologies. Relatively acceptable/pleasing form factors similar to those of well accepted lighting products may be desirable while maintaining advantages of solid state white lighting, such as relatively high dependability, long life and efficient electrical drive of the solid state light emitters.